Magnetic resonance imaging (MRI) is a recent but extremely powerful noninvasive diagnostic tool. MRI utilizes a combination of a powerful static magnetic field and radio frequency pulses which gather information concerning the location and interrelation of atomic nuclei which possess unpaired electron spin within the body. As hydrogen is the most prevalent element to possess unpaired spin, MRI mainly images hydrogen ion concentration. Based upon this information, a computer is able to generate an anatomic image of the subject. For particular studies, MRI has a distinct advantage over computed tomography (CT) scans. For example, it is presently established that MRI is the diagnostic tool of choice in evaluating the posterior fossa, an anatomical location that is poorly visualized by CT. MRI is also superior to CT in delineating extremity soft-tissue tumors and primary bone malignancies. Whereas CT scans a region of interest in one plane, MRI permits imaging in any desired plane, thus more easily permitting multidimensional mapping of tumors.
These advantages of MRI make it attractive for use in radiation treatment planning. Over the past several years, CT has been used for this purpose and has revolutionized radiation treatment by making available more detailed information concerning tumor localization than was ever before possible: (E. Hart, "The Role of the CT Scanner in RT Planning" 54(613) Radiotherapy 20, 1988.) Still, as suggested above, certain anatomical studies are better suited to MRI, and thus, MRI should potentially complement CT in radiation treatment planning. It has also been suggested that MRI may be synergistic with CT in the definition of tumor volume for a number of disease states. (A. Lichter and B. Fraass, "Recent Advances in Radiotherapy Treatment Planning" Oncoloy, May 1988, p. 43)
In order for these expectations to be met, there is a need to develop a means to accurately interface MRI with other diagnostic imaging modalities such as CT or positron emission tomography (PET) and to transfer tumor localization data obtained from MRI and the other imaging modalities to radiation treatment devices. It is important to realize that due to spatial and temporal magnetic field fluctuations within the MRI field, the displayed image is distorted to varying degrees in a non-uniform manner. These fluctuations are dependent on multiple factors such as ambient temperature, and extraneous magnetic fields in the immediate scanner vicinity. Images appearing on the viewing screen (CRT), and ultimately on the film hardcopy, are the result of system software manipulations intended for viewer aesthetics. Further, the bony skeleton which is often used as a reference in determining tumor location and size with other imaging modalities is not well visualized on MRI. Thus, MRI does not permit direct tumor measurement with the degree of consistency and precision demanded in a treatment planning setting.